Extensive use has been made of a computerized numerically controlled lathe whereby workpieces can be cut into a desired shape with little involvement of the operator in the cutting process. The conventional lathe includes, among other components, a bed and a carriage unit slantly mounted on the bed for sliding movement therealong relative to a workpiece held by the chuck of a spindle. The carriage unit serves as a platform for supporting a tool post which is usually composed of a turret holding a variety of cutting tools.
A z-axis servo motor is used to cause sliding movement of the carriage unit in a z-axis direction parallel to the axis of the spindle. The rotating movement of the servo motor is converted into the sliding movement of the carriage unit by a combination of z-axis ball screw and z-axis ball nut. The z-axis ball screw is rotatably journalled on the bed so that it can rotate in a forward or reverse direction depending on the direction of rotation of the servo motor coupled to one end of the z-axis ball screw. Threadedly engaged with the z-axis ball screw and fixedly secured to the carriage unit, the z-axis ball nut is adapted to displace between the opposite ends of the z-axis ball screw together with the carriage unit, in response to the forward or reverse rotation of the z-axis ball screw. Displacement of the carriage unit in the z-axis direction enables the cutting tool of the tool post to move along the length of the workpiece.
Typically, the prior art carriage unit consists of a saddle body slidably mounted on the lathe bed, a cross slider cradled on the saddle body for sliding movement in an x-axis direction substantially perpendicular to the axis of the spindle and an x-axis servo motor attached to one end of the saddle body for causing the cross slider to move in the x-axis direction. The rotating movement of the x-axis servo motor is converted into the sliding movement of the cross slider by means of a combination of x-axis ball screw and x-axis ball nut. The x-axis ball screw is rotatably journalled on the saddle body and coupled at one and to the x-axis servo motor by way of a torque limiter clutch. The x-axis ball nut is threadedly engaged with the x-axis ball screw and fixedly secured to the underside of the cross slider, whereby the x-axis ball nut and hence the cross slider are caused to displace between the opposite ends of the x-axis ball screw as the x-axis ball screw rotates in the forward or reverse direction. Displacement of the cross slider in the x-axis direction enables the cutting tool of the tool post to move toward or away from the workpiece.
A number of drawbacks are noted in accordance with the above-mentioned and other prior art carriage units. First of all, the carriage unit requires to employ a relatively large number of parts, which may result in a reduced manufacturability, a decreased structural integrity and a lowered degree of precision. Secondly, the x-axis ball screw in the carriage unit has a tendency to suffer significant change in its length and position due to the thermal expansion and the inherent weakness of the bearing housing attachment. This may adversely affect the degree of precision which has to be strictly controlled in a computerized numerically controlled lathe. The third shortcoming is that the cross slider may strike and injure the environmental parts at the end of its movement because no means is provided to restrain the overrunning movement of the cross slider.